Caption: Proofreading by DNA polymerase, animation. DNA (deoxyribonucleic acid) is made up of a chain of many paired molecules, known as nucleotides. During DNA replication this chain is split in half lengthwise into two strands. New nucleotides are then paired with each strand, forming two new DNA molecules, each containing one of the original DNA strands. Mistakes in this process are very rare, mainly due to the action of DNA polymerase (blue). DNA polymerase moves along the single strand of DNA building the new strand as it goes. Here DNA polymerase is shown rejecting a non-complimentary base cytosine (C) which will not bind correctly to the base thymine (T). This base is then replaced with a adenine base (A) and DNA replication continues.